Decomposition reactions of dimethyl-di-µ-methylene-bis(η-pentamethylcyclopentadienyl)dirhodium and their relation to the mechanism of the Fischer-Tropsch reactions; the formation of propylene from three C1 ligands

Abstract

Thermal decomposition of [{C₅(CH₃)₅Rh}₂(µ-CH₂)₂(CH₃)₂](1) at temperatures from 275 to 375 °C yielded methane, propylene, ethylene, and some ethane. Using (1) selectively labelled with ¹³C in the Rh–methylene and/or the Rh–methyl ligands showed that (a) the gases are formed in intramolecular decompositions not involving the C₅(CH₃)₅ rings, (b) the methane arises from both the rhodium–methyls and the rhodium–methylenes, and (c) the C₂ and C₃ gases arise predominantly from the coupling of a Rh–methyl and one or two Rh–methylenes; direct coupling of two methylenes or of two methyls is not a favourable process here. Very similar gas mixtures are formed (but at 20–50 °C) on reaction of complex (1) with excess IrCl₆^2–(or other one-electron oxidisers and electrophiles). Carbon-13 and deuterium labelling studies show that these reactions are again intramolecular and do not involve the C₅(CH₃)₅ rings, or the coupling of two methyl or two methylene ligands. Methane arises mainly by combination of a Rh–CH₃ and a methylene hydrogen, probably after a two-electron oxidation, leaving a transient species (A) formulated as [{C₅(CH₃)₅Rh}₂(µ-CH₂)(µ-CH)(CH₃)]²⁺. The C₂ products must be formed from (A) by the coupling of the Rh–CH₃ with the methylene, to give an Rh–ethyl intermediate which β-eliminates to give ethylene (or acquires a hydrogen to give ethane). The labelling shows propylene to arise from the coupling of one methyl and two methylenes. It can be formed via migration of the methyl onto the µ-methyne in (A), giving a µ-methylene-µ-ethylidene species which couples to give propylene directly. Implicit in the route is the need for two metal centres to allow three C₁ fragments to couple together to form propylene. Labelling studies rule out appreciable ethylene formation from a Rh–ethylidene. Direct coupling of the methylenes does occur in the thermal decompositions of [{C₅(CH₃)₅Rh}₂(µ-CH₂)₂X₂](X = halide, SCN, or N₃), to give ethylene; the main product is again methane. The data are contrasted with results from the decomposition of the iridium analogue of (1) and of [C₅(CH₃)₅Ir(CH₃)₄]. The relationships of the mechanisms proposed to current models for the mechanism of the Fischer-Tropsch reaction on metal surfaces are discussed.